This desire is felt when designing certain telemetry systems using wires, radio waves, light waves or other medium to link sending and receiving stations. This desire is felt, for example, where there is a much greater number of sending stations than receiving stations, owing to the overall savings of digital hardware that can then be made in the system.
However, in certain circumstances this desire may be felt irrespective of the relative numbers of sending and receiving stations in the telemetry system owing to the power, volume or weight restrictions imposed upon the sending station being more restrictive than ones for corresponding parameters impose upon the receiving station. Examples of this are where the sending station is in a missile, in a launch vehicle or in an artificial satellite of a planet.
In order to conserve telemetry bandwidth and possibly to reduce average power, it may be desirable to subsample the samples of a digital electric signal descriptive of a measured parameter. To avoid objectionable aliasing being introduced by such subsampling, it is the common practice to filter the digital signal prior to subsampling if the Nyquist rate to properly sample the signal exceeds the subsampling rate. After filtering, the subsampling rate exceeds the new Nyquist rate for the filtered signal, and the subsamples are transmitted to the sending station.
In some applications, phase distortions introduced by filters with non-linear phase responses tend undesirably to obscure features of the telemetry signal. In such cases, the filters used in prior art telemetry have been of finite-impulse-response (FIR) type in order to secure linear-phase filtering. FIR filters are non-recursive and tend to involve a larger amount of digital hardware than recursive filters--that is, filters of infinite-impulse-response (IIR) type. Extending the number of samples in the filter impulse response by recursion allows more abrupt cut-off to be obtained for the same computation load. Narrower passbands or stopbands can be realized recursively for a fixed amount of power or hardware complexity. A bandwidth limiting filter with sharper cut-off permits the filter response to be subsampled closer to Nyquist limit (i.e., less frequently) without incurring aliasing.
Recursive filters accumulate samples, adding each new sample as weighted by a respective factor less than unity, to an accumulation of past weighted samples, which permits their structures to be relatively simple while their impulse response is long-extended. Usually only a single multiplier is used for each accumulation procedure, and each procedure generates an impulse response of extended duration in terms of number of samples. This extended impulse response is obtained through short term storage of accumulation results. There is no need for extensive delay network and a large number of multipliers to obtain such extended duration of response as would be the case in an FIR filter. The simpler IIR filter structures tend to use less power and have less volume and weight than FIR filter structures when impulse responses are required over a large number of sample intervals.
However, the distortion in phase attendant with the use of IIR filters has led engineers away from using recursive pre-filters for subsampling telemetric data. Correction of these phase distortions at the sending station has been done, but the attendant increase in the amount of filtering at the sending station also increases the complexity of the overall filter. In any case, the desire for maximization of complexity and power at the sending station is not achieved.